1. Field of the Invention
The invention relates to a discharge lamp using an anode of the water-cooled type.
2. Description of Related Art
For a discharge lamp with high output power, for example, for a light source of an "artificial sun" device, such as in the development of the universe and in research of the universe or the like, generally an anode of the water-cooled type is used. In such a lamp with a high output power, an anode and a cathode are placed opposite one another inside a bulb made of quartz glass and, at least inside the anode, a cooling water duct is made. This duct is placed inside an element that is a main part of the anode. At a tip of this element, tungsten is attached to receive the arc discharge produced between the electrodes, and between them a brazing filler metal is used as a connecting material.
Materials of varying type are used for such an anode. For the element, copper or nickel with good electrical conductivity and good heat conductivity is used, and for the brazing filler metal, protective layers of nickel are used.
It is true that such a water cooling arrangement is required for an anode whose temperature is slightly raised. But, it is possible for the cathode also to have such an arrangement. Such water-cooled electrodes are used, for example, for a lamp with a high output power of 10 KW to 30 KW. Further, in the case where the cathode is also has a water cooling arrangement, the tungsten can be mixed with thorium.
The conventional water-cooled anode has the following drawbacks:
(1) The brazing filler metal used to connect the element with the tungsten on its tip is, with respect to heat conductivity, not always sufficient. The heat produced in the tungsten at the tip is, therefore, not conducted into the element to a sufficient extent. The heat produced in the tungsten thus vaporizes the above-described tungsten material, which then adheres to the inside of the luminous discharge tube, producing a blackening that contributes to a diminishing of the light emission. PA0 (2) Fissures appear in the tungsten at the tip when the temperature increases. If the fissures occur topically, the temperature increases still more, which causes not only an acceleration of the vaporization of the above-described tungsten, but also causes the arc discharge to become unstable. PA0 (3) Corrosion progresses from the cooling water duct inside the element toward the tungsten at the tip, and needle-fine holes penetrate through the element and the tungsten on its tip. This produces the drawback that, through these holes, the cooling water gets into the interior space of the luminous discharge tube, or the like. PA0 (1) Because of the direct connection of the element and the tungsten to one another, without a brazing filler metal of poor heat conductivity, the tungsten can be advantageously cooled. Thus, the temperature increase of the tungsten and, further, the fissuring of the tungsten can be prevented. PA0 (2) By fixing the thickness of the tungsten in the lengthwise direction of the element, there is no problem in the connection, even if a direct connection without is produced without using the brazing filler metal. PA0 (3) The insulating film attached on the inner side of the tip of the element can prevent corrosion inside the element.